U.S. patent number 4,957,952 [Application Number 07/485,773] was granted by the patent office on 1990-09-18 for anionic electrocoat compositions containing epoxy phosphates.
This patent grant is currently assigned to DeSoto, Inc.. Invention is credited to Aurelio Parenti, Kazys Sekmakas.
United States Patent |
4,957,952 |
Sekmakas , et al. |
September 18, 1990 |
Anionic electrocoat compositions containing epoxy phosphates
Abstract
Thermosetting aqueous anionic electrocoating composition are
disclosed which comprise a carboxyl-functional anionic polymer
dispersed in water with the aid of water miscible organic solvent
and a volatile base, and from about 1% to about 5%, based on the
weight of the polymer content of the composition, of an
oxirane-free epoxy phosphate, this epoxy phosphate being produced
by slowly and incrementally adding a resinous polyepoxide to a
solvent solution containing from 0.05 to 0.9 mole of
orthophosphoric acid per oxirane equivalent in the polyepoxide
together with sufficient water to hydrolyze all of the oxirane
functionality which is not consumed by the orthophosphoric acid.
These epoxy phosphates improve the corrosion resistance of
electrodeposited cured coatings without objectionably wrinkling
their surface.
Inventors: |
Sekmakas; Kazys (St. Petersburg
Beach, FL), Parenti; Aurelio (Norridge, IL) |
Assignee: |
DeSoto, Inc. (Des Plaines,
IL)
|
Family
ID: |
27048474 |
Appl.
No.: |
07/485,773 |
Filed: |
February 21, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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847535 |
Apr 3, 1986 |
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Current U.S.
Class: |
523/402; 523/409;
523/412 |
Current CPC
Class: |
C09D
5/4407 (20130101); C09D 5/4484 (20130101) |
Current International
Class: |
C09D
5/44 (20060101); C08G 059/14 (); C08L 063/00 () |
Field of
Search: |
;523/402,409,412 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Michl; Paul R.
Assistant Examiner: Doody; Patrick A.
Attorney, Agent or Firm: Dressler, Goldsmith, Shore, Sutker
& Milnamow, Ltd.
Parent Case Text
This application is a continuation of application Ser. No.
06/847,535, filed Apr. 3, 1986 now abandoned.
Claims
What is claimed is:
1. A thermosetting aqueous anionic electrocoating composition
comprising, a carboxyl-functional anionic polymer dispersed in
water with the aid of water miscible organic solvent and a volatile
base, and from about 1% to about 5%, based on the weight of the
polymer content of the composition, of an oxirane-free epoxy
phosphate, said epoxy phosphate being produced by slowly and
incrementally adding a resinous polyepoxide to a solvent solution
containing from 0.05 to 0.9 mole of orthophosphoric acid per
oxirane equivalent in said polyepoxide together with sufficient
water to hydrolyze all of the oxirane functionality which is not
consumed by said orthophosphoric acid.
2. An aqueous anionic electrocoating composition as recited in
claim 1 in which said epoxy phosphate is present in an amount of
from 2% to 4%.
3. An aqueous anionic electrocoating composition as recited in
claim 1 in which said polyepoxide is a diglycidyl ether of a
bisphenol having a 1,2-epoxy equivalency in the range of 1.3-2.0
and an average molecular weight of from about 500 up to about 5000,
and orthophosphoric acid is used in an amount of from 0.1 to 0.7
mole of ortho phosphoric acid per oxirane equivalent in said
polyepoxide.
4. An aqueous anionic electrocoating composition as recited in
claim 1 in which said polyepoxide is a diglycidyl ether of a
bisphenol having a 1,2-epoxy equivalency in the range of from 1.7
to 2.0 and an average molecular weight of from about 600 up to
about 3000, and orthophosphoric acid is used in an amount of from
0.2 to 0.5 mole of ortho phosphoric acid per oxirane equivalent in
said polyepoxide.
5. An aqueous anionic electrocoating composition as recited in
claim 1 in which said carboxyl-functional anionic polymer is a
copolymer of monoethylenically unsaturated monomers comprising
sufficient carboxy monomer to enable dispersion in water with the
aid of a volatile base, and sufficient hydroxy monomer to enable
cure with a methylol-functional curing agent.
6. An aqueous anionic electrocoating composition as recited in
claim 5 in which said monomers comprise from 5% to 20% of acrylic
or methacrylic acid and from 2% to 20% of hydroxy monomer.
7. An aqueous anionic electrocoating composition as recited in
claim 5 in which said methylol-functional curing agent is an
aminoplast resin.
8. An aqueous anionic electrocoating composition as recited in
claim 7 in which said aminoplast resin is a hexamethoxymethyl
melamine.
9. An aqueous anionic electrocoating composition as recited in
claim 1 in which said epoxy phosphate is present in an amount of
from 2% to 4%.
Description
DESCRIPTION
1. Technical Field
This invention relates to anionic electrocoat compositions adapted
to deposit coatings exhibiting improved corrosion resistance.
2. Background of the Invention
Carboxyl-functional polymers, and especially carboxyl-functional
acrylic copolymers, can be dispersed in water with the aid of a
volatile base, such as ammonia or a volatile amine, which forms a
salt with the carboxyl groups in the polymer. These aqueous
dispersions at appropriate low solids content for electrocoating
(from 3% to 20%, more preferably from 5% to 15%) are
electrodeposited at the anode of a unidirectional electrical
system. A curing agent, typically a phenoplast or an aminoplast
resin, is incorporated into the aqueous composition and
electrodeposited together with the carboxyl polymer. When the
deposited coatings are baked, highly useful cured coatings are
obtained. However, the corrosion resistance of these known aqueous
anionic electrocoat compositions is less than desired, and it is
the intention of this invention to improve this corrosion
resistance, especially as indicated by salt spray testing.
DISCLOSURE OF INVENTION
In accordance with this invention, the known aqueous anionic
thermosetting electrocoating compositions which comprise a
carboxyl-functional polymer containing hydroxy groups for cure and
which is dispersed in water with the aid of a water miscible
organic solvent and a volatile base are modified for improved
corrosion resistance by the inclusion in the composition of from
about 1% to about 5%, preferably from 2% to 4%, of certain
oxirane-free epoxy phosphates. When these modified cationic
coatings compositions are electrodeposited at the anode and cured
by baking at appropriate temperature, it is found that the
corrosion resistance of the cured coatings is enhanced without
excessively disturbing the smoothness of the coating obtained in
the absence of the added epoxy phosphate. Gloss is reduced somewhat
in this invention, but gloss is a matter of preference. The more
epoxy phosphate used herein, the lower the gloss, so epoxy
phosphate content can be used in this invention for gloss control.
On the other hand, even 1% of the previously used epoxy phosphates,
or more than 5% of the epoxy phosphates used herein, produce
wrinkles in the surface of the cured coatings, and such surface
nonuniformity is unacceptable.
The carboxyl-functional polymer can be self-curing, as by the
presence of N-methylol groups therein. When the polymer is not
self-curing, a curing agent therefor is incorporated into the
composition.
Aqueous coating systems in which carboxy-functional copolymers
containing hydroxy groups are dispersed in water with the aid of
water miscible organic solvent and a volatile amine and cured by
the presence of a water soluble or water dispersible aminoplast or
phenoplast curing agent are well known. Efforts have previously
been made, as described in Kazys Sekmakas and Raj Shah U.S. Pat.
No. 4,461,857 which is commonly owned with this application, to
improve the corrosion resistance properties of the coatings by the
incorporation of epoxy phosphates in electrodeposited coatings.
Unfortunately, the epoxy phosphates described in U.S. Pat. No.
4,461,857 unacceptably degrade the surface smoothness of the cured
anodic electrodeposited coatings before they are used in a large
enough proportion to provide an useful improvement in corrosion
resistance, so they are not practicably used in such coatings. The
epoxy phosphates which are used in this invention are not the same
as those used in U.S. Pat. No. 4,461,857.
The oxirane-free epoxy phosphates used herein are provided by
reacting a resinous polyepoxide with from 0.05 to 0.9 mole,
preferably from 0.1 to 0.7 mole, and most preferably from 0.2 to
0.5 mole of orthophosphoric acid per equivalent of oxirane in the
polyepoxide using a process in which a water miscible organic
solvent in admixture with orthophosphoric acid (which contains a
limited amount of water) is heated to reaction temperature together
with an amount of water sufficient to hydrolyze that portion of the
oxirane functionality in the polyepoxide which does not react with
the phosphoric acid. The resinous polyepoxide is then slowly and
incrementally added to the heated mixture so that reaction with
phosphoric acid and hydrolysis of the oxirane groups will consume
the added epoxy functionality quickly and thus minimize this
functionality in the reaction mixture as the reaction proceeds.
This minimizes epoxy-epoxy reactions which increase the molecular
weight of the product and impair the capacity of the epoxy
phosphate which is produced to be used in the anodic electrocoat
compositions under consideration.
The proportion of water can be increased above the minimum
specified above and may exceed the equivalents of polyepoxide, as
illustrated in Example 1 hereinafter. The amount of water is
preferably sufficient to consume at least about 50% and more
preferably at least about 75% of the oxirane functionality in the
epoxy resin reactant.
The temperature of reaction for the production of the hydrolyzed
epoxy phosphates can vary from about 80.degree. C. to about
130.degree. C. Under these moderate conditions, the reaction is
limited to essentially only one of the three OH groups in the
orthophosphoric acid. It is preferred to use a relatively high
boiling solvent, like 2-butoxy ethanol, and to use reaction
temperatures near the boiling point of water, e.g., 90.degree. C.
to 105.degree. C.
While any water miscible organic solvent can be used, like acetone,
butanol, isopropanol, and the like, the ether alcohols illustrated
by the preferred 2-butoxy ethanol, are preferred. In the presence
of the phosphoric acid, no catalyst is needed and the desired
epoxy-consuming reactions proceed without it. In a preferred
embodiment, more than 75 percent of the organic solvent is
2-butoxyethanol.
The presence of the phosphoric acid provides acidity which can be
measured. This acidity remains in the films which are
electrodeposited and may help to disperse the epoxy phosphate
solutions which are produced herein in the aqueous electrocoating
baths which they are added to. Also, the phosphoric acid groups
catalyze the cure, especially when an aminoplast resin is the
curing agent. The amount of phosphoric acid used is adjusted to
provide the desired water dispersibility and cure enhancement, and
these factors will vary with the polyepoxide selected, the
proportion of solvent and the cure which is desired.
While orthophosphoric acid is usually used, pyrophosphoric acid is
considered an equivalent because it generates orthophosphoric
acid.
Any organic solvent-soluble resinous polyepoxide may be used
herein. By a polyepoxide is meant an epoxide having a 1,2-epoxy
equivalency of at least about 1.2. Diepoxides are preferred,
especially diglycidyl ethers of bisphenols having a 1,2-epoxy
equivalency in the range of 1.3-2.0. The class of bisphenols is
well known, and bisphenol A is usually used in commerce. Diglycidyl
ethers of bisphenol A are commonly available in commerce and such
commercial materials may be used herein. These may have a molecular
weight of from about 350 to about 8,000. It is preferred to employ
those polyepoxides having a 1,2-epoxy equivalency of from 1.7-2.0
and an average molecular weight (by calculation) of from about 500
up to about 5000. A molecular weight of from about 600 to about
3,000 is particularly preferred. Epon 1004 from Shell Chemical
Company, Houston, TX, is useful herein. Epon 1001 (also available
from Shell) further illustrates suitable polyepoxides, and is
preferred.
The term "a bisphenol" is known to describe compounds of the
formula: ##STR1## in which X is a straight chain or branched chain
divalent aliphatic radical of from 1 to 3 carbon atoms, of
>SO.sub.2, >SO, or --O--.
The preferred bisphenol is bisphenol A
(4,4'-isopropylidenediphenol) in which X is 2,2-propylidene and the
two hydroxyl groups are in the para position. Other suitable
bisphenols include 4,4'-thiodiphenol and 4,4'-sulfonyldiphenol.
A preferred embodiment of the invention will be described using a
preferred diglycidyl ether of bisphenol A having a number average
molecular weight of about 1000.
As previously noted, the reaction with the stoichiometric
deficiency of orthophosphoric acid leaves some of the epoxy groups
unreacted. These unreacted epoxy groups are preferably hydrolyzed
with water present in the reaction mixture when the polyepoxide is
added thereto. However, there is usually some alcohol present in
the reaction mixture, so some esterification with alcohol may take
place. These unreacted epoxy groups lead to instability in the
aqueous dispersions which are formed, so any significant proportion
thereof cannot be tolerated.
In conventional electrocoating practice, grounded conductive
objects are immersed in the electrocoating bath and a
unidirectional electrical current is passed through the bath and
through the grounded object as anode to cause the carboxyl
polymers, curing agent and any pigment dispersed in the bath to be
electrodeposited upon the anode.
The voltages used for electrodeposition, the washing procedures
employed to rinse off the bath material which remains on the
electrocoated object (which is usually ferrous metal) and the
baking conditions generally applicable to the various carboxyl
polymer systems in use, are all known in the art and are
illustrated in the example of preferred practice herein.
The copolymers used herein are solvent soluble copolymers of
monoethylenically unsaturated monomers including
carboxyl-functional monomer providing carboxyl groups which enable
dispersion in water and hydroxy functional material providing
hydroxy functionality for cure. The bulk of the polymer is provided
by nonreactive monomers like styrene, vinyl toluene, methyl
methacrylate, and the like. Copolymerization in organic solvent
solution is preferred to provide copolymers having an hydroxyl
value in the range of 30 to 300, preferably from 50 to 150, and an
acid number of at least about 10. Acid numbers of 12 to 60 are
preferred for electrodeposition, but higher acid numbers up to
about 100 may be used.
The carboxyl functional monomers are illustrated by acrylic and
methacrylic acids, though the entire class of monoethylenic
monomers carrying one or more carboxyl groups can be used, such as
crotonic acid, monobutyl maleate and maleic or fumaric acid.
Hydroxy functionality may be introduced using monomeric or
polymeric materials or by generating it after polymerization. The
useful monomers are illustrated by 2-hydroxyethyl acrylate and
2-hydroxypropyl methacrylate, though other monomers carrying one or
more hydroxy groups can be used, such as allyl or methallyl
alcohol. It is also possible to react a portion of the carboxy
functionality provided by the carboxy functional monomer with a
monoepoxide to generate the hydroxy monomer after the copolymer has
been formed. Suitable monoepoxides are 1,2-propylene oxide,
1,2-butylene oxide, epoxy olefins obtained by epoxidizing a
C.sub.12 -C.sub.16 olefin, and epoxy esters, such as the commercial
product Cardura E.
When hydroxy functional polymeric materials are used, one may
employ epoxy ethers and esters. It has been found that ethers of
oleyl alcohol with a diglycidyl ether of bisphenol A having a
number average molecular weight of about 1000, to consume all the
epoxy functionality, provides an hydroxy functional diether which
is quite useful in providing corrosion-resistant electrocoat
products, but even these are benefitted somewhat by this invention,
especially their resistance to detergents. Also, similar
unsaturated epoxy ethers and esters can be used in place of the
oleyl ethers noted above, albeit the oleyl ethers are significantly
better. These epoxy derivatives are copolymerized with
monoethylenic monomers, including carboxyl-functional monomers, to
provide a copolymer which can be used for anodic
electrocoating.
The monoethylenically unsaturated monomers are preferably
copolymerized in water-miscible organic solvent solution to provide
the soluble copolymers which are primarily contemplated herein,
these will include "nonreactive" monomers, as previously noted, and
will usually also include reactive monomers unless reactive groups
are supplied by some higher molecular weight entity with which the
monomers are polymerized, such as the epoxy resin derivatives noted
previously. The purpose is to provide polymer containing hydroxy
groups which can be used for cure, either because they are reactive
under normal curing conditions with themselves or other groups in
the polymer, or because they are reactive under normal curing
conditions with reactive groups supplied by an extraneous curing
agent such as the aminoplast and phenoplast resins which have been
noted.
The term "nonreactive" as applied to a monomer denotes the absence
in the monomer of functional groups, other than the single
polymerizable unsaturated group, which will react under the
contemplated conditions of polymerization and cure. Normally, this
means that the single ethylenic group is the only potentially
reactive group present. In the preferred practice of this
invention, 20% to 45% of styrene and/or vinyl toluene is combined
with from 25% to 50% of alkyl acrylate or methacrylate, such as
n-butyl acrylate or ethyl hexyl methacrylate, enough hydroxy
monomer, such as 2-hydroxyethyl acrylate, to provide reactivity for
subsequent cure (usually from 2% to 20%), and a sufficient
proportion of monoethylenic carboxylic acid to provide
dispersibility in water with the aid of a volatile amine (usually
from 5% to 20%) and the water miscible organic solvent in which the
solution copolymer was prepared. Preferred acrylic copolymers are
illustrated in the Examples.
Reactive monomers may provide cure in the absence of an external
cross-linking agent, or they may require such external agent for
cure. Monomers which enable cure in the absence of an external
agent are illustrated by isobutoxymethyl acrylamide or
isobutoxymethyl methacrylamide. These may be replaced by other
alkyl ethers of N-methylol acrylamide or methacrylamide, such as
the hexyl or octyl ethers.
When an N-methylol functional monomer is included within the
carboxyl-functional polymer, it is desirably used in an amount of
from 5% to 40% of the copolymer, preferably from 20% to 35%, and
even when it is used, an external curing agent may still be used,
albeit in smaller amount than if the N-methylol functional monomer
were not included within the amine-functional polymer.
The aminoplast and phenoplast curing agents may be water soluble or
water dispersible and are themselves well known and commonly used
for the curing of reactive copolymers of the type under
consideration. These are used in an amount of from 5% to 40%,
preferably from 10% to 35%, based on total resin solids. Aminoplast
resins are preferred, such as hexamethyoxymethyl melamine.
Urea-formaldehyde condensates and benzoguanamine-formaldehyde
condensates are also useful. Useful phenoplast resins are
illustrated by a phenol-formaldehyde A-stage resol, and also by
water insoluble heat-hardening phenolic resins which are
dispersible in the copolymer dispersions under consideration.
All proportions herein and in the accompanying claims are by
weight, unless otherwise specified.
EXAMPLE 1
1320 grams of 2-butoxy ethanol, 94.4 grams of 85% ortho phosphoric
acid and 130 grams of additional water are heated to 95.degree. C.
in a reactor and then 2100 grams of a diglycidyl ether of bisphenol
A having a number average molecular weight of 1000 (Shell Chemical
Company product Epon 1001 may be used) are added slowly over 30
minutes. The temperature is then held at 95.degree. C. for 3 hours
to insure completion of all the reactions (with the phosphoric acid
present and with the water). Then 480 grams of additional 2-butoxy
ethanol are added to dilute the product to 55.1% solids content.
The solution product has a Gardner-Holdt viscosity of Y-Z and an
acid value (based on the nonvolatiles) of 41.5.
EXAMPLE 2
An acrylic copolymer solution which is useful in anodic electrocoat
is provided at 60% nonvolatile solids content and containing 40.56%
styrene, 44.27% n-butyl acrylate, and 14% of acrylic acid (a
portion of which is post-reacted with 1.17% of propylene oxide to
convert a portion of the acrylic acid to the hydroxy propyl ester.
This acrylic copolymer is polymerized in conventional fashion at
about 80% solids and then diluted to 60% solids by the addition of
more solvent and diisopropyl amine which catalyzes the
esterification reaction with the propylene oxide. The copolymer
solution has a Gardner viscosity of U-X, a Gardner color of 3 and
an acid value of 82. The volatile portion of the solution contains
37.1% of 2-butoxy ethanol, 2.06% diisopropyl amine and 60.83% of
isopropanol.
EXAMPLE 3
A pigment paste is formed by mixing together in a sand mill 545
pounds of the acrylic copolymer solution of Example 2, 193 pounds
of 2-butoxy ethanol, 74 pounds of 85% diisopropyl amine 40 pounds
of fumed silica, and 744 pounds of titanium dioxide, rutile. Sand
milling is continued to a Hegman grind gauge rating of 7.
EXAMPLE 4
The pigment paste produced in Example 3 is let down by mixing with
450 pounds of a partially butylated hexamethoxymethyl melamine
(American Cyanamid product Cymel 1130 may be used), 100 pounds of
the epoxy phosphate product of Example 1, 24 pounds of 2-butoxy
ethanol, and 307 pounds of 85% diisopropyl amine. The resulting
concentrate is saleable and is mixed with deionized water by the
customer to provide an anodic electrocoating bath.
EXAMPLE 5
The concentrate of Example 4 is mixed with deionized water with
stirring until the total solids content was in the range of 9-10%
solids and the water-dispersed concentrate at 90.degree. F. was
electrodeposited upon steel panels which were immersed in the bath
and electrically connected to constitute the anode of a
unidirectional electrical current at 150 volts. Electrodeposition
was allowed to take place until the current flow substantially
ceased. At that time, the coated panels were removed from the bath,
rinsed with deionized water and then baked for 20 minutes at
375.degree. F.
The steel panels were iron phosphate-treated cold rolled steel
panels (Parker Chemical Company designation EP-10).
The cured coatings were moderately glossy and exhibited excellent
salt spray resistance in that there was hardly any through film
corrosion and only about 1/8 inch creepage from the scribe cut in
the salt spray testing which will be described in conjunction with
the comparative tests which will now be discussed.
Several anodic electrocoat baths were made as here described with
the exception that a bath A was made without any added epoxy
phosphate, a second bath B was made with 2% of the epoxy phosphate,
a third bath C was made with 4% of the epoxy phosphate, and a
fourth bath D was made with 6% of the epoxy phosphate. All of these
proportions are by weight based on the total weight of polymer.
These baths were electrodeposited on EP-10 panels using a bath
temperature of 90.degree. F. and a deposition voltage of 170 volts
for 90 seconds. After rinsing and curing for 20 minutes at
350.degree. F. the deposited films ranged in thickness from 0.7i5
mil to 0.85 mil.
The respective cured coatings were close in appearance, but when
cut to base metal and subjected to continuous salt spray in a dark
chamber at 95.degree. F. for 400 hours (ASTM test B 117), the
coatings lacking the epoxy phosphate rusted and the rust crept away
from the cut line about 1/4th inch to 5/8 inch and through film
corrosion was estimated to be about 5-10%. For many utilities this
is unacceptable. The coatings containing 2% epoxy phosphate showed
less rusting (averaging 3/8th inch creep from the cut) and through
film corrosion was reduced somewhat to an average of about 1-2%.
The coatings containing 4% epoxy phosphate showed an average of
only about 1/8th inch creep from the cut and through film corrosion
was eliminated. This is a remarkable improvement. The corrosion
results at 6% epoxy phosphate content were very favorable, but now
the films exhibited significant surface wrinkling and were not
satisfactory.
Similar favorable results were obtained using zinc
phosphate-treated cold rolled steel panels, albeit the improvement
was not as great. The iron phosphate treatment, however, is a more
practical and less costly pretreatment. Also, and using oleyl
alcohol epoxy ether copolymers the detergent resistance was
improved and the catalytic action of the epoxy phosphate allowed a
reduction in curing temperature.
* * * * *